1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically but not exclusively, to carrier-phase estimation and data recovery for coherent optical receivers.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Delivery of multimedia services (e.g., telephony, digital video, and data) that is implemented using optical phase-shift keying (PSK) or quadrature-amplitude modulation (QAM) signals has certain advantages, e.g., over that implemented using conventional electrical analog or digital signals. As a result, cable companies are upgrading their hybrid fiber coaxial networks to improve/create fully interactive, bidirectional optical networks that can carry optical multimedia signals into and out of homes. It is projected that, in the near future, high-definition television signals are likely to be delivered exclusively over optical communication channels.
A typical coherent optical receiver detects the received optical communication signal by mixing it with a local-oscillator (LO) signal and then processing the resulting mixed signals to determine the phase and amplitude of the communication signal in each time slot (symbol period), thereby recovering the encoded data. To enable this phase and amplitude determination, the LO signal may be phase-locked to the carrier frequency (wavelength) of the communication signal using an optical phase-lock loop (PLL). More specifically, the PLL is configured to track the frequency and phase of the communication signal and provide a feedback signal to the LO source, based on which the LO source achieves and maintains the phase lock.
Unfortunately, suitable coherent optical receivers are typically relatively difficult to design and/or relatively expensive to build. For example, a conventional, relatively inexpensive laser source might produce an optical signal that has a relatively large linewidth. If that laser source is used in a coherent optical receiver as a local oscillator, then its relatively large linewidth might produce a phase uncertainty/noise that can make the optical phase-lock between the LO and communication signals difficult to achieve and/or maintain. As a result, coherent optical receivers are often designed to have specially constructed laser sources and/or relatively complex optical PLLs, both of which can drive up the receiver cost by a substantial amount.